Exoplanet hunters are missing 75 per cent of two-star worlds

Binaries are twice the trouble. The shifty geometry of planets that orbit two stars means we’ve missed about 75 per cent of these worlds – but we are playing catch-up.

Planets that orbit two stars are truly, intriguingly alien – they have varied seasons, and formed under different circumstances from planets in our own solar system. They are also trickier to discover and study: unlike planets around single stars, they shift their orbital paths over just a few years.

The Kepler telescope has spotted 10 of these worlds by watching them transit, or cross in front of their stars from our point of view. Transits around just one star run like clockwork: once you know how long the planet’s year is, you can predict exactly when it should next pass in front of the star.

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But binaries have more moving parts. The planet could orbit its stars in the same plane, or adopt its own separate plane. The angle of its orbit with respect to the stars’ plane shifts with each trip around the binary. Sometimes the planet will transit as it goes around. Sometimes it won’t.

Since the easiest way to find the planet is when it transits, we really need to know when to look – a tricky problem only solved by number crunching on high-powered computers.

An easier way

In 2015, David Martin at the Geneva Observatory in Switzerland started looking for an easier way. First, he and his colleagues devised equations to calculate whether a circumbinary planet would transit at all – an easier technique than running simulations.

Now, he has calculated when a planet’s orbit crosses in front of the orbital path of its stars. That gives astronomers specific windows in which they have a good chance of spotting a transit.

Martin also quantified how lucky Kepler was to see those 10 planets during its four-year mission, given that they don’t always transit. Extrapolating that chance suggests that 30 more similar planets may lurk unseen in the same systems.

“Using the results here we can estimate how many we miss, making it much easier to understand the whole population,” says David Armstrong at the University of Warwick in Coventry, UK, who previously estimated that about 10 per cent of binary stars have close-in gas giant planets – about the same fraction as single stars.

Next, Martin hopes to pinpoint future transits precisely.

“Calculating that probability is bloody hard,” he says. “I’m trying to do it but I haven’t figured it out yet.”